When plastic part has external thread features, demolding generally adopts opening cavity, ejecting plastic part from core to realize automatic demolding, which is conducive to simplifying mold structure; When demoulding is required in same direction as axial direction of external thread or in vertical direction, a special side core pulling mechanism needs to be designed for side core pulling and demolding. Layout of plastic part to be molded in mold opening plane is closely related to selection of mold release method. In the case that demolding can be completed, ayout of plastic part to be molded in mold should be considered in favor of simplifying mold structure, so as to reduce mold manufacturing cost and ensure reliability of mold mechanism.

 

Figure 1 Plastic parts
Plastic part is a special-shaped bottle cap, as shown in Figure 1, external dimension is 82.4mm* 46.5mm*37.2mm, structure is a double-layer cylindrical cap. Main body is composed of an outer wall and an inner straight pipe, a hollow inner groove is formed between outer wall and inner straight pipe. Extension section of inner straight pipe at the other end of main body is an elbow pipe with external threads. An elastic buckle is provided on ridge line of outer wall, and base of buckle is provided with arrayed ribs. In order to ensure elasticity of buckle, except for bottom of buckle, two sides adjacent to outer wall are provided with notches. According to environmental requirements of plastic parts, PC modified ABS alloy with a shrinkage rate of 0.5% is selected for plastic parts.

As shown in Figure 1, when designing parting of plastic parts, ease of demoulding of plastic parts, difficulty of processing and manufacturing of molded parts, reliability of mold work should be considered. There are two ways to set type of plastic parts. First solution is a vertical layout. Molded part on the side of inner straight tube of molding is set as core insert of cavity. Outer wall and outer thread of molding use Haval sliders, which require double-sided core pulling and demolding. Forming elbow section needs to adopt elbow core pulling mechanism on the side of cavity. Under this layout, height of mold increases, when Haval slider mechanism on movable mold side is formed, as working time of mold increases, parting surface where Haval slider meets will wear out, resulting in flashing of molded plastic part, and manufacturing cost of Haval slider is higher.
In addition, elbow core pulling mechanism is set on the side of fixed mold, which increases height of fixed mold, runner of mold will be lengthened, which is not conducive to injection of melt into cavity, and it is difficult to set elbow mechanism on the side of fixed mold. Therefore, first typing method is not desirable. Second scheme adopts a horizontal layout, that is, plastic part to be formed is arranged on mold opening plane according to PL parting surface shown in Figure 1. External thread and outer wall can be automatically demolded by opening movable mold and fixed mold. Inner straight pipe and inner groove can be formed by using a side slider to complete forming and demoulding in 2 times. First time is used for demoulding of inner groove, and second time is used for demoulding of inner straight pipe; Section can be formed and demolded by adopting a horizontal layout of elbow core pulling mechanism, buckle position can be formed and demolded by a separate side core pulling slider.
Demolding mechanism of second scheme is all arranged on mold opening surface, which is beneficial to simplify mold structure, reduce processing difficulty of mold parts, improve reliability of mechanism and convenience of maintenance. Final part of plastic part adopts second scheme, considering space required by mechanism design and production quantity requirements, cavity is determined to be 2 cavities. Gating system adopts a side gate, which is set on ridge line of outer wall of plastic part to be molded (see Figure 1), and gate is rectangular.

 

(A) Cavity layout

 

(B) Demoulding mechanism
Figure 2 Cavity layout and demoulding mechanism design
1. Arc slider 2. Plastic parts 3. Side core 4. Inner core 5. Side core 6. Chute pull block 7. Sector drive block 8. Rotary shaft 9. Rack 10. Core insert 11. Side slider
According to above analysis, layout of cavity and demolding mechanism is shown in Figure 2. 2-cavity layout adopts a back-to-back form, which is beneficial to use a central core-pulling chute pull block to synchronously drive two side inner cores for core-pulling, completing molding and demolding of plastic part's buckle feature; elbow section adopts an arc slider for rotating core pulling, and bottom side slider through which arc slider passes drives 2 racks, thereby driving sector drive block to rotate around shaft to realize rotating core pulling of elbow core; demolding of straight pipe section and groove is also completed by a common side core slider drive.

 

Figure 3 Mold structure
1. Fixed mold base plate 2. Fixed mold plate 3. Movable mold plate 4. Support plate 5. Cushion block 6. Movable mold base plate 7. Push plate 8. Push rod fixed plate 9. Pull rod sleeve 10. Travel switch 11. Fan-shaped drive Block 12. Key 13. Rotating shaft 14. Drive rod 15. Limit screw 16. Curved groove bead 17. Arc slider 18. Elbow core 19. Cavity plate insert 20. Left slider 21. Left slider Block limit bead 22. Chute pull block 23. Right slider limit bead 24. Right slider 25. Core insert 26. Water nozzle 27. Guide sleeve 28. Guide column 29. Rack 30. Teeth Bar slider 31. Rack hydraulic cylinder 32. Push plate guide column 33. Push rod 34. Support column 35. Limit block 36. Limit nail 37. Return spring 38. Reset rod 39. Wear plate 40. Hydraulic cylinder 41. Hydraulic cylinder backing plate 42. Piston rod bushing block 43. Straight tube core pulling slider 44. Groove core cover plate 45. Straight tube core 46. Groove core 47. Inclined sprue sleeve 48. Positioning Ring seal
Designed mold structure is shown in Figure 3. Mold base is a two-plate mold structure, and two kinds of guides are used to guide moving parts of mold. One is movement guidance when movable mold plate 3 and fixed mold plate 2 of mold are closed. Guide post 28 and guide sleeve 27 are used for guiding. There are 4 sets of guide post and guide sleeves. Guide post and guide sleeve on the side of reference angle of mold is installed eccentrically to prevent misalignment of movable mold and fixed mold from damaging molded parts and mechanical parts of mold during assembly. Second is movement guidance of ejector plate of mold, which is guided by push plate guide post 32 and corresponding ejector plate guide sleeve. There are also 4 sets of guides, which are respectively installed at four corners of ejector plate. Function of adding support column 34 is to enhance support strength of support plate 4 on mold base, prevent injection pressure transmitted by core insert from causing deformation of movable mold plate.
Cavity plate insert 19 and core insert 25 of molded part are respectively installed in grooves of fixed mold plate 2 and movable mold plate 3, fastened by screws. Datum of cavity plate insert 19 and core insert 25 is lower right corner datum method, which is same as datum method of mold base to reduce mold system error caused by inconsistent design datum, processing datum and assembly datum. Material of cavity plate insert 19 needs to have good hardness and strength, good polishing performance. Its material is S136, and heat treatment hardness is 47~50HRC. Insert is integral, that is, it is directly milled from blank, and local difficult-to-machine areas are inlaid with small inserts to reduce processing difficulty. Core insert 25 is made of 45 steel, its heat treatment hardness is 38~42HRC, and it is also directly milled from whole blank.
Molded parts of inner wall of plastic part to be molded are inlaid with a straight tube core 45, a groove core 46, and an elbow core 18. Molded parts of side wall buckle are right slider 24 and left slider 20. Materials of two sliders are imported alloy steel material S136 with higher hardness and better processing performance.
For cooling of molded parts, layout of cooling pipes adopts a balanced layout to ensure that plastic parts can obtain maximum possible uniform cooling. Diameter of cooling pipes is ϕ8mm, and cavity plate insert 19 is cooled by two water channels arranged on fixed mold plate 2 and core insert 25 is cooled by three water channels arranged on movable mold plate 3.

 

(A) Sealing ring

 

(B) Gate
Figure 4 Sealing and gate design
In order to ensure that water channel does not leak water in gap between molding insert and mold plate, water channel uses a 2.5mm sealing ring shown in Figure 4 (a). Single-point pouring is used for a single cavity, gates in two cavities use shrink-type side gate shown in Figure 4(b), and runner of gating system uses a ϕ6mm circular runner. In order to reduce length and width of mold base, main runner adopts an inclined sprue sleeve 47 for gating.
Molded plastic part is finally pushed out by push rod. Push-out combination plate composed of push plate 7, push rod fixing plate 8 and pull rod sleeve 9 are fastened by screws. Pull rod of injection molding machine directly pulls pull rod sleeve 9 to drive push plate to push out and reset. There are 4 reset springs 37, which are distributed on reset rods 38 at four corners of mold push plate to ensure a balanced reset of push plate. Function of reset rod 38 is to ensure that push plate must be reset and retracted when mold is closed.

 

(A) Straight tube secondary delay side core pulling mechanism

 

(B) Elbow core pulling mechanism
Figure 5 Demoulding mechanism
Demolding mechanism is shown in Figure 5. Secondary side core pulling mechanism for straight tube, elbow core pulling mechanism, and double side retracting core pulling mechanism are main demolding mechanisms for forming plastic part. Secondary delay side core pulling mechanism of straight pipe is used for partial demolding of straight pipe wall and groove of straight pipe section of plastic part to be molded; forming and demolding of inner wall of elbow section of plastic part to be molded is implemented by elbow core pulling mechanism; release of buckle feature of plastic part to be formed is implemented by a double-side retracting core-pulling side core-pulling mechanism.
(1) Components of secondary delay side core pulling mechanism of straight pipe include parts 39~46, as shown in Figure 5(a), in molded parts of straight pipe section of plastic part, groove core 46 is concentrically sleeved on the outside of straight pipe core 45, groove core 46 is installed on straight tube core-pulling slider 43 through groove core cover plate 44. There is a 10mm delay distance between shoulder of tail end of straight tube core 45 and top of corresponding sleeve of groove core 46 to ensure that groove core 46 is pulled out by 10mm before straight tube core 45 is removed from inner wall of straight tube of plastic part. Working principle of mechanism is: hydraulic cylinder 40 drives straight tube core pulling slider 43 to pull core through piston rod clamping block 42. When straight tube core pulling slider 43 moves outward, it first drives groove core 46 to pull out 10mm. After that, straight tube core 45 is driven to be drawn out from inner wall of straight tube of plastic part to reduce single core pulling force and prevent plastic part from being deformed when it is demolded.
(2) Pieces 29 to 31 and pieces 11 to 18 constitute elbow core pulling mechanism. Rack 29 is mounted on rack slider 30, the two are fastened by screws. Rack hydraulic cylinder 31 is used to drive rack 29. Rack hydraulic cylinder 31 is installed under movable mold plate 3 as a power component. Rack 29 is used to drive sector-shaped drive block 11 for rotation; sector-shaped drive block 11 is sleeved on rotating shaft 13 and is connected to rotating shaft 13 through key 12. Rotating shaft 13 is installed in corresponding shaft hole on movable mold plate 3 to ensure that fan-shaped drive block 11 can be driven by rack 29 to rotate around rotating shaft 13; fan-shaped drive block 11 is provided with a drive rod 14 through which upper end of drive rod 14 is inserted into a hole in the center of circular arc slider 17. When fan-shaped drive block 11 rotates, it can synchronously drive arc slider 17 to rotate, driving elbow core 18 to rotate and pull core. Arc slider 17 guided by curved groove pressing bar 16, and arc stroke is limited by limit screw 15. Elbow core pulling mechanism is shown in Figure 5(b).
(3) Double-side retracting core-pulling mechanism is composed of pieces 20-24. Chute pull block 22 is fastened on fixed mold plate 2 by screws, right slider 24 and left slider 20 are driven by oblique T-shaped grooves on both sides to retract core and complete side core pull of undercut on ridge line of outer wall of plastic part. During demolding, core-pulling strokes of right slider 24 and left slider 20 are controlled by right slider limit bead 23 and left slider limit bead 21 installed in core insert 25.

(1) Mold installation and injection. After mold is hoisted on injection molding machine, mold is closed, and nozzle of injection molding machine is aligned with sprue sleeve for injection. After injection is completed, mold is opened after pressure is maintained and cooled.
(2) Mold opening. Movable mold of mold is driven by ejector rod of injection molding machine, movable mold and fixed mold are opened at PL parting surface by pulling force of elbow of injection molding machine. When opening,chute pull block 22 drives right slider 24 and left slider 20 to retract core, first perform core-pulling and demolding on the undercut of outer wall; At the same time, fixed mold plate 2 unlocks straight tube core pulling slider 43 and circular arc slider 17.
(3) After mold is opened, hydraulic cylinder 40 drives secondary delay side core pulling mechanism of straight pipe to complete core pulling of plastic elbow section, and then rack hydraulic cylinder 31 drives to complete rotary core pulling of plastic elbow section.
(4) Launch. When movable mold retreats to a certain distance, combined board of push plate 7 and push rod fixed plate 8 is supported by ejector pin of injection molding machine, movable mold plate and core insert 25 on it continue to retreat. Push rod on composite plate pushes plastic part out of core insert 25, and plastic part is completely released mold.
(5) Reset. When resetting, injection molding motorized mold plate pushes movable mold forward. Under elastic force of reset spring 37, core insert 25 moves upward to realize resetting of push rod, then hydraulic cylinder 40 piston rod drives straight tube secondary delay side core pulling mechanism, movable mold continues to move, mold is closed at PL parting surface. During closing process, right slider 24 and left slider 20 are driven and reset by chute pull block 22.
In view of special structure of plastic part, difficulty of demolding inner and outer walls, a 2-cavity injection mold was designed. Injection mold is equipped with a straight tube secondary delay side core-pulling mechanism, an elbow core-pulling mechanism, and a double-side retracting core-pulling mechanism to realize demolding of three features of plastic part to be molded. Structural layout of corresponding mechanism and auxiliary parts in mold is reasonable, layout and structural design of demolding mechanism are ingenious and reasonable, which can provide a useful reference for molding of similar plastic parts.